Heating Problem on 1992 Dodge Caravan

When the engine warms up the fan does'nt always come on.I relaced the thermostat,radiator cap,fan motor and flused out the fluid.But it still over heats but only when I deliver mail.I had two mechanics look at it and neither could get it to overheat.One suspects blown head gasket.Any sugestions would be appreciated.

Your cooling fan is controlled by a temperature sensor and a cooling fan relay. I would scan the data stream and watch the activity of the temp sensor and the 'cooling fan' on data point. Do you have to add coolant on a regular basis? Good Luck!